CFRP materials are increasingly used in new aircraft developments. In order to use them it is necessary to adapt external influencing factors, such as temperature and humidity, to the material-specific properties so the strength behaviour is only affected to a minimal extent.
Modern aircraft air-conditioning systems (fresh air supply systems) operate by the principle of single-stage compression by a compressor that is driven by an air turbine. The ram air is used to cool the hot and compressed engine bleed air through the external flow during flight and through a fan in the ram air channel at ground level. This air in turn drives the compressor via a turbine and is conditioned in accordance with the requirements in the cabin.
The purpose of the ram air channel of the fresh air supply system is to make the external air available as cooling air for the heat exchanger. The ram air channel generally consists of a NACA ram air inlet channel, a diffuser, a rubber hose connection, possibly a ram air channel plenum and a ram air outlet channel. The heat exchanger and the plenum of the fresh air supply system are installed between the ram air channel plenum and the ram air outlet channel with an ACM fan (ACM=air cycle machine (three-wheel turbomachine, i.e. turbine, compressor, fan)). The ACM fan and the compressor are driven by the expansion in a turbine. The ACM fan ensures that the cooling air is guided through the heat exchanger, even at ground level.
During flight the external flow arrives at the ram air channel via the ram air inlet channel, which is usually of NACA shape. Some of the dynamic portion of the total pressure is converted into the static portion in the diffuser (the flow decelerates). Static overpressure (relative to the ambient pressure) is thus produced and is also called ram pressure at the entrance to the heat exchanger. The circulation of cooling air is controlled by two movable, interconnected ram air inlet channel flaps. In this instance the front flap in the direction of flight is rigidly attached to the frame and is moved up/down at the end via a lever of a spindle. The second flap is connected to the first flap at the end of the first flap via a hinge. The upward/downward movement (i.e. the closing or opening of the ram air inlet) results in parallel displacement of the rear end of the second flap, said displacement being compensated for by a short pendulum rod.
The ram air outlet channel is generally equipped with only one flap. The opened ram air outlet channel flap generates a vacuum in the ram air outlet channel as a result of the circulation by external air. This vacuum affects the cooling mass flow through the heat exchanger. The flap is operated by an actuator.
In the method described the cold ram air that flows through the heat exchanger via the ram air inlet channel is heated to such an extent that the outlet exhaust air temperature can lie in the range up to 200° C. During flight this hot exhaust air may be applied to the surface of the aircraft by the effect of the external flow. Despite specific cooling by the external flow, the temperature can still be up to 160° C. when it reaches the aircraft CFRP structure. The strengths of current CFRP materials can still be quantitatively determined up to a temperature range of 100° C. to 120° C., depending on the load profile and ambient conditions. Loads above this temperature range may lead to irreversible material damage and therefore to breakdown.
A current solution for reducing the outlet temperature provides a ‘cooling film’ that, similarly to a cold air cushion, acts between the hot exhaust air and the surface of the aircraft, as described for example in DE 102 44 199 A1. The cooling film keeps the temperature of the surface of the aircraft below the critical temperature for CFRP. The cooling film is produced as a result of the pressure differential between the installation space for the aircraft air-conditioning system and the outside environment. A strong vacuum region is produced during flight at the surface of the aircraft in the region of the ram air channel owing to the position of the ram air outlet channel at the surface of the aircraft. The source of the cooling film is ventilation air that is made available from the separate ram air inlet. The cooling film is guided selectively beneath the exhaust air flow.
The main drawback of this principle is that when the cooling film fails, for example as a result of the impact of a foreign body or specific flight manoeuvres, the wing box may overheat in an unperceived manner since the aircraft air-conditioning system is still in operation. An electric shutdown function is problematic with regard to safety requirements.
The cooling air film also cannot be maintained during every flight phase. The wing flaps and/or spoilers/airbrakes are extended during the landing approach and during low-speed flight in such a way that the above-mentioned installation space for the aircraft air-conditioning system is connected to the outside environment. There is a stronger vacuum in the regions of the wing flaps and spoilers/airbrakes than in the region of the ram air outlet channel in such a way that the cooling air does not flow out through the slot for the cooling air film, but instead flows out in the region of the wing flaps and spoilers/airbrakes. The cooling film thus provides no protection against hot exhaust air during these flight phases. There is also no protection against hot exhaust air in the case of a fault, for example damage to the surface of the aircraft, which may lead to failure of the cooling air film.
One object of the present invention is therefore to provide a device and a method that make it possible to cool the hot exhaust air from a fresh air supply system as required in the ram air outlet channel, in such a way that the aircraft CFRP structure arranged behind the channel is subjected to low exhaust air temperatures.
Accordingly, a bypass channel that bypasses an aircraft air-conditioning system arranged downstream of a ram air channel with a ram air inlet channel portion, and merges into an outlet channel, arranged downstream of the aircraft air-conditioning system, before a discharge opening for exhaust air is provided in a device according to the invention for exhaust air cooling of aircraft air-conditioning systems, a common inlet being provided for the ram air channel and the bypass channel, is provided.
In the present invention the exhaust air is already cooled before exiting from the outlet via a ram air bypass channel to such an extent that the critical temperature for CFRP cannot be reached. If the ram air bypass channel fails, the aircraft air-conditioning system also fails or is switched off in such a way that a fault will not result in overheating of the CFRP aircraft structure. In this instance both the ram air channel and a variable bypass channel are supplied by a single air inlet.
The bypass channel is preferably connected as directly as possible to the aircraft air-conditioning system outlet channel in order to keep the pressure loss low. It preferably merges into the aircraft air-conditioning system outlet channel before the discharge of the exhaust air into the outside environment. The channel thus bypasses the aircraft air-conditioning system and feeds cold ram air into the hot exhaust air flow.
A common inlet is preferably provided for the ram air and the bypass channel. This has the advantage that the cooling film is always provided during operation of the aircraft air-conditioning system.
The variable permeability of the bypass channel is achieved by a bypass channel flap. In this constructional solution the parallel displacement of the second flap of the ram air inlet channel is utilised in order to adjust the inlet opening of the bypass channel. Instead of the pendulum rod, an inlet flap of the bypass channel assumes the function of the pendulum rod and simultaneously adjusts the opening cross-section of the bypass channel. The bypass channel is opened when the flaps of the ram air inlet channel are closed. The bypass channel is closed to a maximum when the flaps of the ram air inlet channel are arranged in the maximum ram air channel flap position for flight. The end of the second flap of the ram air inlet channel forms the removal point of cold air for the bypass channel.
The purely mechanical adjustment operates by the following principle. If a high cooling power of the aircraft air-conditioning system is required, the ram air inlet channel is opened to the maximum (flaps move downwards). For this cooling situation the maximum available ram air is required by the heat exchanger to cool down the bleed air. The design is configured in such a way that the opening of the bypass channel is closed as far as possible (flap of the bypass channel is closed, permeability of the bypass channel is low). Since in this instance the ram pressure in the ram air channel is high, the mass flow through the bypass channel will be sufficient to provide the protective function. In addition, a large cooling air mass flow is provided for this operating state and therefore the exhaust air temperature does not fall within the critical temperature range. A large bypass cooling flow is therefore also not necessary.
By comparison, minimal heat is to be emitted by the bleed air during heating. The ram air inlet channel is therefore moved to the minimum opening (flaps move upwards). In this instance the ram pressure in the ram air channel is low so the high permeability of the bypass channel is required in order to ensure a sufficient mass flow through the bypass channel. In this operating state the low volume of ram air is additionally well heated by the bleed air in such a way that the increased bypass cooling air flow is required. This design is configured in such a way that the opening of the bypass channel is opened as far as possible (permeability of the bypass channel is maximal).
The fan installed for ground operations scoops the ambient air toward the heat exchangers and generates a vacuum in the ram air inlet channel. Hot exhaust air can thus be supplied to the inlet via the bypass channel, as a result of which the exhaust air temperature can be heated up to the critical range. A non-return valve is thus provided for this situation.
The constructional integration of the bypass channel and of the ram air channel in an air inlet makes it possible to achieve a clear safety advantage over a separate bypass channel. If the NACA inlet becomes damaged; for example as a result of the impact of a bird, it is more likely that, in addition to the bypass channel, the aircraft air-conditioning system will simultaneously also no longer function. In this instance no more hot exhaust air will reach the aircraft structure. By comparison, with a separate bypass inlet the aircraft air-conditioning system would continue to operate via its own channel in such a way that the hot exhaust air can reach the aircraft structure unimpeded and damage the material thereof in a sustained manner. The purely mechanical construction offers greater reliability compared to electrical solutions (sensors, additional electrical flap control, etc.).
In the figures, like reference numerals denote like or functionally identical components, unless indicated otherwise.